In the related art, research results that have been obtained show that the number and the size of ACF (aberrant crypt foci) formed in recta have a strong correlation with a future incidence of cancer. In such research, a method is employed, in which, for example, the pit pattern is made visible using methylene blue, and the ACF are checked one-by-one while counting the number thereof by using a magnifying endoscope or the like (for example, see Non Patent Literature 1).